The present invention relates to an electromechanical converter for use in actuators of industrial robots, ultrasound motors, etc., and more particularly to an improvement in laminate-type displacement elements constituted by a plurality of thin plates made of electromechanical converting materials laminated with internal electrodes for having larger displacement amounts.
Conventionally, laminate-type displacement elements used in positioning mechanisms and brakes of XY stages are constituted by thin plates made of piezoelectric ceramic materials and having predetermined shapes which are provided with electrodes and polarized and bonded to each other directly or via thin metal plates with an organic adhesive. However, in the laminate-type displacement elements in which the thin plates are laminated by an adhesive, displacement by the vibration of the piezoelectric elements is absorbed by the adhesive under certain conditions, or the adhesive is deteriorated in a high-temperature environment or by use for a long period of time.
Recently, laminate-type piezoelectric elements of such a structure as laminate-type chip capacitors have been put to practical use. Specifically, as described in Japanese Patent Publication No. 59-32040, starting material powder is mixed with a binder to provide a piezoelectric ceramic material paste and formed into a thin plate having a predetermined thickness, and it is coated with an electrically conductive material such as silver-palladium, etc, on one or both sides thereof to form an internal electrode. Predetermined numbers of the thin plates thus coated are laminated, pressed and worked in a predetermined shape and then sintered, and the resulting laminate is equipped with outer electrodes on both sides. The laminate-type piezoelectric element of the above structure is excellent in the adhesion between the thin plates made of a piezoelectric ceramic material and the internal electrodes, and has excellent thermal stability which makes it endurable for use at high temperature and minimizes the deterioration thereof in use for a long period of time.
This laminate-type piezoelectric element, however, has various problems in a piezoelectric displacement efficiency and other points. For instance, as shown in FIG. 11, the laminate-type piezoelectric element is constituted by a plurality of thin plates 1, 1, made of a piezoelectric ceramic material which are laminated alternately with positive and negative internal electrodes 2a, 2b, and each of the internal electrodes 2a, 2b is connected to each external electrode 3a, 3b. In this case, a deflecting portion A is constituted by the plurality of thin plates 1, 1, each coated with the internal electrode, and a non-deflecting portion B is formed in the periphery portion of the laminate constituted by the thin plates 1, 1, where the plurality of internal electrodes 2a, 2b are not attached the thin plates 1, 1, Because of this structure, when voltage is applied to the external electrodes 3a, 3b, an electric field intensity is large only in the deflecting portion A and it is small in the non-deflecting portion B, resulting in hindered deformation of the piezoelectric element. Thus, large displacement inherent in the piezoelectric ceramic material cannot be achieved. In addition, since large strain is concentrated on the boundary between the deflecting portion A and the non-deflecting portion B, the thin plates or the overall element is likely to be broken.
To solve this problem, the applicant proposed a laminate-type piezoelectric element having a structure shown in FIG. 12 (Japanese Patent Laid-Open No. 62-200778). In FIG. 12, the same reference numerals are assigned to the same parts or portions as in FIG. 11. Each thin plate is formed in a shape having a pair of projections 1a on both sides, so that a deflecting portion A in which internal electrodes 2a, 2b are laminated with the thin plates 1 is relatively large. This structure increases the displacement by the same driving voltage by 20-30% as compared to the structure in FIG. 11. However, since it still contains a non-deflecting portion B, it is impossible to completely eliminate the problem that the thin plates are broken along the boundary between the deflecting portion A and the non-deflecting portion B.
As an alternative structure which can prevent the decrease in piezoelectric conversion efficiency due to the existence of the non-deflecting portion B shown in FIGS. 11 and 12, a laminate-type piezoelectric element shown in FIG. 13 was proposed (Japanese Patent Laid-Open No. 58-196068). In FIG. 13, the same reference numerals are assigned to the same parts or portions as in FIGS. 11 and 12.
In FIG. 13, a predetermined number of the internal electrodes 2a, 2b are laminated alternately with thin plates 1 with their overall surfaces covered by the internal electrodes 2a, 2b. A pair of insulating members 4 are attached to both sides of the laminate having the above structure in such a manner that they are in contact with the end of every other internal electrode 2a, 2b (only one internal electrode 2a, 2b on each side), and an external electrode 3a made of a conductive material is attached to each of them. By this structure, the internal electrodes 2a, 2b can exist on the entire surface of each thin plate 1, maximizing the deflecting portion A in the element.
However, since this structure requires that the insulating members 4 be applied to the overall width of the internal electrodes 2a, 2b, complicated procedures for constructing the laminate-type piezoelectric element are needed. In addition, the insulating members 4 should be aligned to the centers of the internal electrodes 2a, 2b. However, since the fluctuations and errors in thickness of the thin plates 1 and/or the internal electrodes 2a, 2b make it difficult to position the insulating members 4 exactly at predetermined locations, insulation breakdown is likely to take place. In addition, when the thin plate 1 is as thin as 100 .mu.m or less, the insulating members 4 which should be applied to every other internal electrode 2a, 2b are liable to cover adjacent internal electrodes 2a, 2b, thereby shutting their connection with the external electrodes 3a, 3b.
As an improvement of the above so-called overall electrode-type laminate-type piezoelectric element, the applicant proposed a laminate-type piezoelectric element shown in FIG. 14 (Japanese Patent Laid-Open No. 62-211974). In FIG. 14, the same reference numerals are assigned to the same parts or portions as in FIGS. 11-13. In FIG. 14, the laminate-type piezoelectric element comprises on one side a pair of insulating members 5, 5 which are provided with external electrodes 3a, 3b having lead portions 6a, 6b. The lead portions 6a, 6b are connected to every other internal electrode 2a, 2b. In this embodiment, the lead portion 6a is electrically connected to the internal electrode 2a, and the lead portion 6b to the internal electrode 2b. This structure is effective for avoiding the insulation breakdown unlike in FIG. 13, but there still remains a difficulty in positioning the lead portions 6a, 6b accurately.